The present invention relates to high voltage power supplies, and more particularly, to high voltage power converter circuits having substantially reduced AC voltage between secondary windings.
Many conventional high voltage DC sources rely upon inductive coupling of energy from a low voltage AC source to secondary coils which include a rectifier assembly for rectifying the AC voltage to produce a DC output. An increased output voltage level is achieved by the transformer principle. That is, the low voltage AC source is connected to a primary coil having a small number of turns and rectifying component are connected to a secondary coil having a large number of turns. The inductive coupling in the high turns ratio secondary increases or “steps-up” the voltage to the high output level.
U.S. Pat. No. 5,631,815 to Cross describes a high voltage power supply in which a high voltage DC output is generated by magnetically coupling energy from a high frequency alternating electrical source to an arrangement of rectifier modules in the output stage. Accordingly, the rectifier modules and the associated magnetic coupling operate to limit the alternating voltages produced in the secondary windings to a level below the Paschen Minimum Voltage for the medium surrounding the rectifiers. This facilitates the use of high frequency alternating voltages as the excitation source for the magnetic structure, and eliminates partial discharges which are a serious insulation problem with alternating circuits at high voltage. High frequency excitation of the magnetic structure is desirable because it reduces the size of the structure required and similarly reduces the cost.
With the addition of many low voltages, which each are less than Paschen minimum or approximately 380 Volts, it is possible to use surface mount technology to construct the rectifier stages and to use printed circuit boards as carriers for the secondary windings and rectifier stages. Advantageously, this arrangement provides a reliable and low cost assembly suitable for operation at high frequency excitation. The modular nature of the arrangement makes it is possible to stack a large number of the voltage generating printed circuit boards to provide very high voltages. Assuming that all printed circuit boards used are identical, it is economical to create very high voltages by using a large number of the voltage generating boards connected in series.
In some known power supply configurations, energy from the excitation source is provided to secondary windings and rectifiers through magnetic coupling in a magnetic circuit or core. This magnetic core is assumed to be at ground potential, and the voltage generating boards are insulated from the magnetic core. For practical purposes, such a design works well for moderately high voltages, for example, up to 200 kV. It becomes difficult and impractical to insulate secondary windings of the supply from a grounded magnetic core at higher voltages. Such issues were long ago encountered when high voltage power supplies were excited with 60 Hz oscillations. To surmount the problems, the magnetic core was segmented, and the segments were insulated from one another via a polymer layer. The segments of the magnetic core were then maintained at a voltage level similar to that of neighboring neighbouring secondaries. This became known as an Isolated Core Transformer (ICT) and was first developed Van der Graaf in the 1940's.
It is well known that even at 60 Hz, problems exist with using an ICT. This is especially true at higher currents. The problems arise out of the segmentation of the magnetic core in the transformer which introduces gaps in the magnetic structure with a permeability essentially that of air. This greatly increases the reluctance of the magnetic structure and results in increased primary current required to produce requisite secondary current. Furthermore, leakage of magnetic flux around gaps results in decreased magnetic flux coupled between primary and distant secondaries. This results in a lower generated voltage per turn on the secondary windings. The leakage flux also gives rise to leakage reactance in the equivalent circuit of the transformer. The leakage reactance produces a drop in output voltage proportional to the load current. Because of this leakage, an ICT is highly likely to exhibit a severe drop in output voltage with load current. Thus, it is difficult to design an ICT for very high voltages because of the loss of magnetic flux in the upper stages.
These inherent problems with ICTs operating at 60 Hz were addressed to an extent by winding more turns on upper secondary coils. While this approach may be suitable for conventional ICTs, it is typically undesirable for the arrangement of voltage generating printed circuit board devices using surface mount technology in the high voltage power supply.
Thus, what is desired is a single layer planar power converter circuit having a winding geometry for processing energy to and from high voltages using multiple windings operating at progressively higher voltages and without the use of a conductor between windings.
Such an approach has significant packaging advantages which make feasible secondaries in the range of approximately 30V/turn rather than nearly 200V/turn in the case of the ICT proposed by Cross. This ten fold increase in secondary turns allows drastic reduction in required magnetic cross section at the gaps and increased gap dimension to ensure reliability of inter-ferrite insulation.